Currently available mobile handsets are compatible with networks such as GSM, 3G, and 4G networks. Each of these networks operates at a different frequency band. Traditionally, multiple separate power amplifiers are used to amplify signals of multiple frequency bands. For instance, two power amplifiers have to be used in a mobile handset operable at two frequency bands. Each of the two power amplifiers are then optimized for each of the two frequency bands. This is done to satisfy a maximum linear output power requirement that is usually different between the frequency bands, depending on the loss between the power amplifiers and an antenna. It has also been observed that the optimum load impedance required for each of the two power amplifiers may vary considerably. Although such an approach will provide optimum performance for each frequency band, it has a disadvantage in terms of increased cost and large size due to the use of two or more power amplifiers.
There have also been devices in which a single power amplifier has been used to amplify signals of two or more frequency bands. In such devices, two switches (in case of two frequency bands) are required to be used for the frequency band selection. This approach reduces the overall cost as it requires fewer active components (e.g., power amplifiers); however, the performance of the power amplifier is compromised for wideband operation. Also, the on-state loss of the switches degrades the overall efficiency of the device. In addition, it has been noted that in such devices a wideband matching network is required for impedance matching, however the power amplifier is not able to achieve different output power requirement for different frequency bands since the wideband matching network will constrain the impedance level that can be achieved over the given bandwidth.
To overcome this problem of the tradeoff between the power amplifier matching network and the bandwidth, in some implementations, each frequency band is provided with its own matching network. In such cases, the switches are inserted between the matching networks and the power amplifier output to facilitate selection of a required frequency band. However, since the switches are generally inserted at low impedance points, the on-state switch loss degrades the efficiency of the overall power amplifier performances.
Therefore, it is desirable to find techniques that reduce the cost and the size of the RF power amplifiers, and at the same time, achieving better overall performance.